A voltage regulator is a power supply that provides one or more regulated output voltages from an input voltage. In a DC-DC voltage regulator, both the input and output voltages are direct current (DC) voltages, as shown in FIG. 1. A conversion ratio, defined as VIN/VOUT, is one of the most important specifications of the regulator. If VIN/VOUT is larger than 1, the voltage regulator is usually called a step-down converter. On the other hand, if VIN/VOUT is smaller than 1, voltage regulator is called a step-up converter. The load circuitry could be any device or devices that consume electric power, such as processor chips, a display, and so on.
A switched-capacitor (SC) DC-DC converter is a commonly used DC-DC voltage regulator. The core circuitry of an SC converter is composed of capacitors and switches that are connected together in a variety of different ways. By switching one or more of the switches on and off in certain patterns, the electric energy is delivered from input to output.
FIG. 2 shows an example of a prior art 2-to-1 SC converter. As suggested by its name, the conversion ratio of this converter is 2-to-1. This 2-to-1 SC converter consists of one capacitor C and four switches SW1-SW4. This 2-to-1 SC converter, as well as most typical SC converters, works in a 2-phase operation. The operation of the 2-to-1 SC converter of FIG. 2 is illustrated in FIG. 3. During phase 1, switch 1 SW1 and switch 4 SW4 are turned ON (closed), and switch 2 SW2 and switch 3 are turned OFF (open). In phase 1, the capacitor C is connected between the input VIN and the output VOUT. The voltage across the capacitor C increases as the capacitor is charged. During phase 2, switch 1 SW1 and switch 4 SW4 are turned OFF (open), and switch 2 SW2 and switch 3 are turned ON (closed). In phase 2, the capacitor C is connected between VOUT and ground. The voltage across the capacitor C decreases as the capacitor C is discharged. Energy is transferred from the input VIN to the output VOUT through the charging/discharging of the capacitor C. On average, the voltage across the capacitor C is about VIN/2, creating a conversion ratio about 2-to-1.
Different configurations of the capacitors and switches may result in different conversion ratios. FIGS. 4A-4C show three exemplary converters, each having a different conversion ratio. The phase 1 switches are depicted as being enclosed by a dashed oval, while the phase 2 switches are not enclosed. FIG. 4A depicts an exemplary 2:1 converter. FIG. 4B depicts an exemplary 3:1 converter. FIG. 4C depicts an exemplary 3:2 converter. It is also important to notice that different configurations of capacitors and switches may achieve the same conversion ratio. This creates different topologies of SC converters. There are advantages and disadvantages for each topology, making different topologies suitable for different applications. But fundamentally, the manner in which the capacitors and switches are connected together and the switching pattern of the switches define the characteristics of the SC converter.
In most SC converters, the capacitor-switch switching cell is broken into multiple small unit cells. This is a well-known and well-accepted design method. These converters are usually called multi-unit or multi-phase converters, and the individual unit cells may be referred to as ladder units. FIG. 5A shows an exemplary multi-unit 2-to-1 SC converter 500 that has 4 unit cells. FIG. 5B shows a schematic diagram of power switch drivers 510 configured to control the switches of the converter 500, and an interleaved clocking diagram 520 for the converter of FIG. 5A. The advantage of a multi-unit converter 500 over a single unit converter (see FIG. 2) is that only one unit may switch at a time, while the capacitors in all other units serve as filters if not switching. This reduces the current/voltage noise created by the switching behavior of the converter 500.
A primary function of a DC-DC converter is to deliver a certain amount of power from input to output at a reference output voltage. So the converter usually uses feedback control logic to make sure that it delivers the right amount of power and creates the correct output voltage. A feedback control logic usually monitors the output voltage/current of the converter, and adjusts the behavior, such as switching frequency, of the converter to meet the application requirements.
FIG. 6 illustrates the concept of the well-known single-bound feedback control logic circuit 600. A clocked comparator 642 monitors a output voltage, VOUT, of the converter 620. VOUT is compared to a reference voltage, VREF, at the edge of the clock. If VOUT is higher than VREF, it means the converter 620 delivers more power than the reference voltage. Or if VOUT is lower than VREF, the converter 620 does not deliver enough power. If VOUT is greater than VREF, the converter 620 is not switching, and VOUT is discharged by a load current ILOAD. When VOUT is detected to be lower than VREF, a pulse is created, namely COMPTRIG, which is the output of the comparator 642, and the interleaving clock generator 644 provides a feedback path causing the power switch drivers 630 to signal one of the SC units in the converter 620 switch to deliver more power to the output VOUT. So the feedback logic changes VOUT and the amount of power being delivered. It is named single-bound control mainly because there is only one reference voltage VREF used in the feedback implementation.
FIG. 7 shows the transient output waveform of an SC converter that solely relies on single-bound control. At heavy load conditions (when load current is high), the load current quickly discharges VOUT before the feedback loop can detect and react, creating large voltage ripple. Therefore, there is a need in the industry to address one or more of the abovementioned shortcomings.